need some advice on x y plot

[Grant Edwards]

On 2005-10-25, Grant Edwards <grante at visi.com> wrote:

> It looks like it.  Though I've used custom tics in the past, it
> was never for time values.  Based on the help from gnuplot, I
> suspect you can get what you want without doing custom tics,
> but rather using the commands
>
>    set xdata time
>    set timefmt
>    set format x
>
> Interestingly, using Unix timestamps creates some sort of
> resolution problems.  The following ought to work but doesn't.
> It appears that there is some sort of resolution problem:
>
> ------------------------------8<------------------------------
> import Gnuplot,time,sys,math
>
> def pause():
>     sys.stdout.write("Press enter to continue: ")
>     sys.stdin.readline()
>
> def fgrid(start,stop,count):
>     for i in xrange(count):
>         yield start + ((stop-start)*i)/count
>
> start = time.time()
>
> xdata = [x for x in fgrid(0,600.0,10)]      # two minutes worth
> ydata = [math.sin(x/100.0) for x in xdata]
> data = Gnuplot.Data(xdata,ydata,with='linespoints',using=(1,2))
> gp = Gnuplot.Gnuplot(debug=1)
> gp.title('Data starting at %s' % time.asctime(time.gmtime(start+xdata[0])))
>
> # x axis will use default tics (seconds since start of run)
> gp.plot(data)
> pause()
>
> # same data with x value as Unix timestamps
>
> xdata = [x+start for x in xdata]
> print xdata
> data = Gnuplot.Data(xdata,ydata,with='linespoints',using=(1,2))
>
> gp('set xdata time')
> gp('set timefmt "%s')
> gp('set format x "%r"')
> gp('set xtics 120')
> gp.plot(data)
> pause()
> ------------------------------8<------------------------------

Yup.  Something in the Gnuplot module is broken.  Here's the
x data I'm passing it:

[1130256529.616158, 1130256589.616158, 1130256649.616158,
1130256709.616158, 1130256769.616158, 1130256829.616158,
1130256889.616158, 1130256949.616158, 1130257009.616158,
1130257069.616158]

And here's what it's passing to gnuplot

1130256512.0 0.0
1130256640.0 0.564642488956
1130256640.0 0.93203908205
1130256768.0 0.97384762764
1130256768.0 0.675463199615
1130256768.0 0.141120001674
1130256896.0 -0.442520439625
1130256896.0 -0.871575772762
1130257024.0 -0.996164619923
1130257024.0 -0.772764503956

It appears that the Gnuplot modules has coerced my data into
single-precision -- thus throwing away most of the resolution
on the x-axis.

-- 
Grant Edwards                   grante             Yow!  Yow!
                                  at               
                               visi.com